Weather Report: Tom Skilling 7-Day Forecast WGN-TV Weather Center

Ask Tom Why

Puddles on my driveway following a rain evaporate much more quickly in the summer than in the winter. Why?

Marshall Heiser

Dear Marshall,

The answer lies in the behavior of water molecules and how that behavior changes at different temperatures.

Water molecules possess considerable kinetic energy--they are in constant, rapid motion. That motion increases as their temperature rises.

Evaporation occurs on a molecule-by-molecule basis when a water molecule that is at the evaporating surface obtains enough energy from random collisions with other molecules to break through the water surface into the air--to evaporate.

Water evaporates more quickly in the summer because molecular motion is greater at higher summer temperatures than at lower winter temperatures.

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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.